Integrated devices are usually formed on substrates which serve mainly as supports for their fabrication. However, the increase in the degree of integration and in the performance levels expected of these devices is leading to an increasingly significant coupling between their performance levels and the characteristics of the substrate on which they are formed. Such is particularly the case with RF devices, which deal with signals with a frequency of between approximately 3 kHz and 300 GHz, which are notably applicable in the field of telecommunications (telephony, Wi-Fi, Bluetooth, etc.).
As an example of device/substrate coupling, the electromagnetic fields, derived from the high-frequency signals that are propagated in the devices, penetrate into the depth of the substrate and interact with any charge carriers that are located therein. The result thereof is a pointless consumption of a portion of the energy of the signal through insertion loss and possible influences between components by crosstalk.
It is therefore particularly important to ensure a suitable match between the electrical characteristics of the substrate and the performance levels expected of the devices.
One usual technique for measuring the electrical characteristics of a substrate for the fabrication of RF devices involves forming a test structure on this substrate by employing the usual microelectronic fabrication means: deposition, masking, etching, etc. The test structure can thus be designed to measure a particular characteristic of the substrate (such as, for example, its resistance, its linearity, its capacitance, its permittivity) and/or characterize its suitability for a particular application considered. The test structure can, for example, consist of coplanar lines in which a signal from a generator is propagated. A signal analyzer is used to identify the power dissipated in the substrate and deduce therefrom the insertion loss characteristic.
The known techniques, like those which have just been described, are not entirely adequate. In fact, they require the implementation of significant means, for example, for the production of the test structures on the substrate, which are slow and costly. These means cannot be automated, or can be automated only to a small extent, and they are, moreover, destructive techniques. They are not therefore suitable for operating a substrate production control or substrate quality control on the input side of a device production line. Each measured characteristic must, in addition, be the subject of a separate test. Finally, these known techniques, although they make it possible to locally measure an electrical characteristic, are not suited to producing a mapping of this characteristic over the entire surface of the substrate. They do not therefore make it possible to accurately characterize this substrate.